Please post your full code and test class if any you created.

I will recommend you to start using trailhead to learn about test classes
1) https://trailhead.salesforce.com/modules/apex_testing

Also please check below post
1) https://developer.salesforce.com/docs/atlas.en-us.apexcode.meta/apexcode/apex_qs_test.htm
2) https://developer.salesforce.com/docs/atlas.en-us.apexcode.meta/apexcode/apex_testing_example.htm

Pleasse check below post sample test class
1) http://amitsalesforce.blogspot.com/2015/06/best-practice-for-test-classes-sample.html

You write a test class for this the same way that you would any other:
- Set up some data for the controller to access (Account, Contact etc)
- Execute a method/methods:- search
- Verify the behaviour with asserts.
 

Please update your Batch job like below

global class DeleteAllTask implements Database.Batchable<Sobject>,schedulable{
         
    global Database.queryLocator start(Database.BatchableContext bc)
	{
		String sSOQL = 'Select Id,Name,Status,Subject From Task Where status =\'Completed\' and subject=\'callback\' ' ;  
		if(test.isRunningTest()){
			sSOQL += ' limit 5 ';
		}
		return Database.getQueryLocator(sSOQL);
    }
    global void execute(Database.BatchableContext bc, List<Task> scope){
		Delete scope;
    }      
        
    global void finish(Database.BatchableContext bc){
    }
    
    global void execute(SchedulableContext sc){
        DeleteAllTask oDeleteAllTask = new DeleteAllTask();
        Database.executeBatch(oDeleteAllTask,50);
    }
}

Your test class should be like below

@isTest 
public class DeleteAllTaskBatchJob 
{
    static testMethod void testMethod1() 
    {
        List<Task> lstTask= new List<Task>();
        for(Integer i=0 ;i <200;i++)
        {
            Task oTask = new Task();
            oTask.Subject ='callback';
			oTask.Status = 'Completed';
			oTask.Priority='low';
			//OTHER REQUIRED FIELDS
            lstTask.add(oTask);
        }
        insert lstTask;
        
        Test.startTest();
		
            DeleteAllTask  obj = new DeleteAllTask ();
            DataBase.executeBatch(obj); 
			
        Test.stopTest();
        // Add Assert here to check result
    }
}

Please follow below salesforce Best Practice for Test Classes :-

1. Test class must start with @isTest annotation if class class version is more than 25
2. Test environment support @testVisible , @testSetUp as well
3. Unit test is to test particular piece of code working properly or not .
4. Unit test method takes no argument ,commit no data to database ,send no email ,flagged with testMethod keyword .
5. To deploy to production at-least 75% code coverage is required
6. System.debug statement are not counted as a part of apex code limit.
7. Test method and test classes are not counted as a part of code limit
9. We should not focus on the  percentage of code coverage ,we should make sure that every use case should covered including positive, negative,bulk and single record .
Single Action -To verify that the the single record produces the correct an expected result .
Bulk action -Any apex record trigger ,class or extension must be invoked for 1-200 records .
Positive behavior : Test every expected behavior occurs through every expected permutation , i,e user filled out every correctly data and not go past the limit .
Negative Testcase :-Not to add future date , Not to specify negative amount.
Restricted User :-Test whether a user with restricted access used in your code .
10. Test class should be annotated with @isTest .
11 . @isTest annotation with test method  is equivalent to testMethod keyword .
12. Test method should static and no void return type .
13. Test class and method default access is private ,no matter to add access specifier .
14. classes with @isTest annotation can't be a interface or enum .
15. Test method code can't be invoked by non test request .
16. Stating with salesforce API 28.0 test method can not reside inside non test classes .
17. @Testvisible annotation to make visible private methods inside test classes.
18. Test method can not be used to test web-service call out . Please use call out mock .
19. You can't  send email from test method.
20.User, profile, organization, AsyncApexjob, Corntrigger, RecordType, ApexClass, ApexComponent ,ApexPage we can access without (seeAllData=true) .
21. SeeAllData=true will not work for API 23 version eailer .
22. Accessing static resource test records in test class e,g List<Account> accList=Test.loadData(Account,SobjectType,'ResourceName').
23. Create TestFactory class with @isTest annotation to exclude from organization code size limit .
24. @testSetup to create test records once in a method  and use in every test method in the test class .
25. We can run unit test by using Salesforce Standard UI,Force.com IDE ,Console ,API.
26. Maximum number of test classes run per 24 hour of period is  not grater of 500 or 10 multiplication of test classes of your organization.
27. As apex runs in system mode so the permission and record sharing are not taken into account . So we need to use system.runAs to enforce record sharing .
28. System.runAs will not enforce user permission or field level permission .
29. Every test to runAs count against the total number of DML issued in the process .


Please let us know if this post will help you